Non-destructive Testing (NDT), in the ideal sense, lets one see inside work-pieces to identify materials or to locate and size defects. All NDT procedures have an illumination source: visible light, ultrasonic waves, gamma rays, electromagnetic waves, etc., and some sort of media on which the illuminated image is projected or recorded: retina, transducers, film, impedance analyzers, etc. It is this projected image, scattered from the defect, that is the basis of NDT.
Not all NDT inspection involves defect discrimination. Many NDT applications involve the inspection of variations of layer or plate thicknesses, which could be important for quality control in a manufacturing setting. Other applications might be the monitoring of changes in the stresses in materials by monitoring changes in their material properties, for instance inspecting the tensile and compressive loads which occur in railway rails due to environmental temperature changes.
There are two distinct aspects of NDT, qualitative and quantitative. Qualitative NDT uses variations from some balanced signal to identify material variations or the detection of a defect. On the other hand, quantitative NDT is concerned with material sorting, locating and sizing of defects, providing measures of depth or material properties. Historically, qualitative NDT has been the most widely used. However, if more information is needed than just the mere presence of cracks or corrosion, then quantitative techniques must be applied.
Defects can be combinations of inclusions, cracks, voids, corrosion or variations
in material properties. To some extent the type of defect determines the NDT
technique to be used, because the defect must scatter the illumination source
in a detectable way. In dye penetrant testing, the dye interacts with the crack
structure on the surface of the material, indicating the extent and possibly
some measure of the density of the corrosion, but all sub-surface defects will
not be found. Eddy-current inspection, on the other hand, can interact with
the entire volume of the crack structure, and therefore contains information
about surface breaking and near-surface corrosion.
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